The hearts from 10 necropsy patients with Ebstein's anomaly of the tri-cuspid valve were examined for abnormalities of the mitral valve. Five patients were under 1 year of age and none had an abnormal mitral valve. The other five patients were ages 18 to 72 years (mean 36 years); all had focal fibrous thickening of the mitral leaflets. Only one of the three patients with mitral valve prolapse has an atrial septal defect (secundum type), indicating that the association of mitral valve prolapse with Ebstein's anomaly is not due to the presence of an atrial septal defect.